The invention relates to a conical ultrasonic wave deflection system having an axially symmetric ultrasonic transducer for ultrasonic microscopy utilizing surface waves and/or Lamb waves in an object.
An ultrasonic point scanning microscope is discussed in J. K. Zieniuk, A. Latuszek, Proc. IEEE Ultrason. Symp. (1986), pp. 1037-1039. In this microscope, a frustoconical sapphire point is mounted on an ultrasonic transducer. The point diameter is approximately 1/4 of the ultrasonic wavelength in the coupling medium. Defined waves are thus not formed in the coupling medium and the arrangement results in overresolution.
A cone angle of 20.degree., a point diameter of 20 .mu.m and an ultrasonic frequency of 30 MHz are discussed in this article for excitation of longitudinal waves in the object. The conical shape serves primarily for making the transition from the large ultrasonic transducer face to a small point.
An arrangement for ultrasonic microscopy utilizing surface waves and spherical ultrasonic lenses is discussed by I. R. Smith et al. in Appl. Phys. Lett. (42), 1983, pp. 411-413.
In the Smith et al. device, in the defocused state, the signal components of the longitudinal waves are suppressed, while ultrasonic waves passing through a narrow ring on the spherical lens impinge on the surface of the object at a suitable angle, the Rayleigh angle, for resonant excitation of surface waves, and generate surface waves, with a circular wave front, which converge to a diffraction-limited surface wave focus. A transmission arrangement including a transmitting and receiving unit having a lens and an ultrasonic transducer is also disclosed.
For a reflecting arrangement, only a semicircular sector of a conventional spherical lens/transducer unit is proposed. Either the lens is divided or a semicircular ultrasonic transducer is provided.
In the Smith et al. device, it is not possible to specify a particular angle of incidence on the surface and thus select, for example, particular modes of surface waves. The fact that in the Smith et al. device only a very small part of the ultrasonic energy which is generated and acts on the object is converted into surface waves, and thus used for signal generation, has led to development of a number of other devices.
An arrangement which is intended to avoid the disadvantages of the Smith et al. device is discussed by B. Nongaillard et al. in J. Appl. Phys. 55 (1984), pp. 75-79. In this device, a cylindrical lens is provided instead of a spherical lens, and the longitudinal axis of the cylindrical lens is inclined with respect to the surface of the object. Cylindrical wave fronts then have an elliptical line of intersection with the surface of the object. If the angle of inclination corresponds to the Rayleigh angle, then surface waves are generated which, however, due to the elliptical generation zone, converge to a line focus on the surface. The required inclination of the cylindrical lens with respect to a perpendicular to the surface of the object is impractical for application to a conventional ultrasonic microscope.
In U.S. Pat. No. 4,779,241, issued to Atalar et al., a plane ultrasonic transducer acts at an oblique angle on a reflecting or diffracting, preferably parabolically cylindrical, face so that conical wave fronts are produced, the cone axis coinciding with the focus line of the reflecting or diffracting face. The objective is arranged perpendicular to this axis, and the intersecting line of the conical wave fronts with the surface of the object is then a circular sector, so that a point focus similar to the Smith et al. device is obtained. By varying the angle between the ultrasonic transducer and the focusing face, the Rayleigh angle for generating surface waves is obtained.
This technique can be carried out for transmission and reflection, with separate or unified transmitting and receiving ultrasonic transducers. However, due to the oblique angle required, it is difficult to make this method compatible with a conventional ultrasonic microscope.
It has also been proposed to utilize the shape of the ultrasonic transducer to achieve a desired wave geometry. An ultrasonic transducer in the form of a cone sector, the cone axis of which is perpendicular to the surface of the object, is discussed by S. Ayter in Proc. of 1987 IEEE Ultrason. Symp., pp. 301-304.
This technique, however, requires a significantly increased manufacturing outlay for making the ultrasonic transducer.
It is known from A. Atalar et al., Proc. 1988 IEEE Ultrason. Symp., pp. 771-774 that the excitation of generalized Lamb waves can be used advantageously for ultrasonic image generation on objects with a layer structure.